1. Field of the Invention
The present invention relates to a servodrive apparatus for use in a postage meter.
Postage meters are used to store large total amounts of postage in a register and, on command, print portions of this total on individual pieces of mail in place of postage stamps. After each postage printing operation the total amount of postage stored in the register is depleted by the amount of postage dispensed.
Since postage dispensed by the meter is in effect currency which may be sold or exchanged, the meter must be carefully controlled to print accurately but to not print if the stored postage has been exhausted. Therefore, after each printing cycle is completed, the register is ordinarily sensed to determine whether or not sufficient postage remains in storage to print the next piece of mail. If there is sufficient postage, the meter is operable to print a desired amount. If not, the meter is "locked out" or rendered inoperative until again supplied with sufficient postage. For example, most postage meters are capable of printing up to $9.99 of postage. Accordingly, when a postage meter contains less than $10.00 of postage, the meter is locked out.
The servodrive apparatus of the present invention is intended for use in a postage meter of the type which employs a rotating drum, having a printing die which prints indicia representative of the specific amounts of postage on the individual pieces of mail. Each piece of mail is transported to the drum by a conveyor. The angular velocity of the drum is chosen so that the speed of the printing die is synchronized with the speed of the mail which passes by it to minimize distortion of the printed postage indicia. Further, passage of mail by the rotating drum is timed so that the postage is printed at the correct location on the mail.
Rotation of the printing drum in such postage meters is ordinarily stopped after each printing cycle to permit the postage storage register to be sensed as described above. That is, in each printing cycle, the printing drum is first accelerated from a home rest position to a constant velocity which is maintained for a predetermined period, during which the individual piece of mail is printed, and then is decelerated back to rest at the home position. Typical constant angular drum velocity in meters intended for high speed operation may be 350 rpm or more.
2. Description of the Prior Art
Rotating printing drum postage meters known in the prior art usually utilize a spring clutch arrangement to drive the drum from the same motor which drives the mail conveyor. Though inexpensive, such spring clutch arrangements accelerate the printing drum to and from constant angular velocity very abruptly. The angular velocity, as a function of time, of a typical postage meter drum driven through a spring clutch is indicated by dotted lines in FIG. 4. As can be seen, this velocity increases to the constant velocity portion of the printing drum cycle in a very short period of time, approximately 25 milliseconds. The angular velocity of the drum decreases from its high constant value in even shorter time, on the order of 10 milliseconds. Accordingly, acceleration and deceleration are very high. Therefore, postage meter operation is characterized by noisy chatter and meter operational life is substantially decreased.
These drawbacks of driving a meter printing drum through a spring clutch by the mail conveyor motor are aggravated when the meter is operated at high speed as described. Therefore, these prior art meters are not well suited for high volume, high speed applications.
Apparatus for controlling machines having incrementally rotating parts are also known. Such an apparatus is shown in FIG. 2 of PMI Application Note AN-4 entitled "Incremental Motion Servos" published by PMI Division of Kollmorgan Corporation. A similar apparatus is shown in FIG. 3 of an article entitled "Motors With Printed Circuit Armatures" by Hans Waagen, Design Engineering, January 1970. However, these apparatus are not adapted to control increasing and decreasing angular component velocity so that it substantially equals a preselected angular velocity.